In many paper making machines, through-air dryers (TADs) are used for effecting evaporative drying of the web either after or instead of pressing devices. Typically a TAD unit includes a hollow rotatable drying roll having a porous cylindrical roll face around which the wet web of paper is partially wrapped as the web is passed through the device. The web is typically supported on a continuous fabric as it is passed through the TAD unit. Heated air passes through the porous roll face and through the web and fabric so as to cause evaporative drying of the web. For reasons of energy efficiency, the heated air may be recovered after it has passed through the web and a substantial portion of the recovered air recirculated back through a heating device where it is reheated and passed back through the porous roll face and the web and fabric.
Because the fabric and web cannot be wrapped a full 360.degree. 0 around the drying roll, in all rotary TAD units there is an inactive sector or dead zone of the drying roll which the fabric and web do not contact. In some TAD units, for example as shown in U.S. Pat. No. 3,303,576, heated air under pressure is supplied to the interior of the roll through the porous roll face in the dead zone and the heated air passes radially outwardly through the porous roll face and through the fabric and web along the active portion of the roll. In this type of TAD unit, the paper web may contact the drying roll and the supporting fabric is on the outer surface of the web. The air after passing through the web is recovered by an exhaust hood which surrounds the active portion of the roll. However, in dryers employing such radially outward flowing drying air, the pressure differential across the web, and hence the air flow and drying capacity, are limited by the fabric tension. Additionally, the supply ducting which passes the heated air through the roll face into the roll interior takes up space adjacent the roll surface and thus conflicts with the objective of maximizing the circumferential extent (i.e., the "wrap angle") of the drying roll about which the web and fabric are wrapped. It is desirable to maximize the wrap angle so as to maximize the drying area and hence the time during which a given portion of the web is on the drying roll. Furthermore, the supply ducting increases the overall height of the TAD unit and thus conflicts with the objective of minimizing the overall height of the TAD unit.
Another type of TAD unit is exemplified in U.S. Pat. No. 3,432,936, wherein the heated air is supplied under moderate pressure to a supply hood which surrounds the active portion of the drying roll, and the heated air is drawn by the exhaust system from the supply hood through the web and fabric and through the porous roll face of the drying roll into the roll interior. In some of the embodiments disclosed in the '936 patent, air is exhausted from the interior of the drying roll through the porous roll face in the dead zone of the roll and into an exhaust plenum which is sealed against the roll face in the dead zone. In such an arrangement, the exhaust plenum conflicts with maximizing the wrap angle of the web, as noted above for the supply duct of the '576 patent. Furthermore, in this configuration, the exhaust ducting increases the overall height of the TAD unit and thus conflicts with the objective of minimizing the height of the TAD unit. The '936 patent also discloses embodiments in which air is exhausted axially through one end wall or head of the drying roll and is routed axially away from the roll by a duct. However, such axial ducts increase the overall axial length of the TAD unit. Also known are TAD units in which air is exhausted axially out both ends of the dryer roll.
In addition to the objectives of maximizing the wrap angle and minimizing the overall height and length dimensions of a TAD unit, it would also be desirable to provide a TAD unit enabling access to the hoods, dryer roll, and the associated guide rolls which guide the continuous fabric (also known in the industry as "clothing") about the dryer roll, so that debris can be removed from the hoods. In an inward-flow TAD unit having hoods located beneath the roll, access to the lower part of the hoods is especially desirable as paper may lodge in the area as a result of process upsets. Additionally, the fabric must be removed and replaced periodically, and thus it is desirable to be able to slip a new continuous fabric axially over the end of the roll. Split hoods formed in two halves which can be moved away from each other and away from the roll have been developed to facilitate access to the internal areas of the hoods and to the dryer roll for inspection and fabric changing purposes. However, a continuing problem with TAD units in which air is directed via ducts axially through the head at the tending side of the roll is the need to be able to disconnect the ducting to gain access to the dryer roll for fabric changing.
TAD units with duct sections that are entirely removable have been developed to provide the needed access to the dryer roll, but such removable ducts tend to be large and unwieldy. Furthermore, removing and replacing a duct section necessitates breaking and re-establishing seals at both ends of the duct section every time a fabric is changed.
An alternative solution to the access problem which has been used in some TAD units of the inverted hood type in which the hood is located essentially beneath the dryer roll is to route the air duct above the dryer roll from the "tending side" of the dryer roll (i.e., the side of the machine opposite from the drive side where the drive motor is located) axially to the drive side of the machine. However, this duct arrangement has its own disadvantages. Specifically, if the duct passes close to the surface of the dryer roll, it tends to interfere with maximizing the wrap angle of the web about the roll; conversely, if the duct is well above the roll surface, the overall height of the machine increases. Additionally, such axially routed ducts tend to be long, creating greater pressure losses in the system.